The present invention relates to a golf ball which, in long-term use, has an excellent durability to cracking, durability of appearance and durability to ball surface loss, which utilizes a waste material and thus is environmentally beneficial and moreover which, even with the use of a waste material, is able to maintain the basic ball performance (feel, flight, durability) at a high level.
In order to ensure the durability of a golf ball in long-term use, it is necessary to enhance, for example, the durability of each member of the ball and the wear resistance of the outside surface. Recently, from an environmental standpoint, the idea of utilizing waste materials by incorporating such materials in golf ball core- or cover-forming materials has begun to emerge in the golf ball industry. The basic performance attributes of golf balls include flight, feel at impact, and durability. When a waste material is included in the core or cover, it is essential to choose the type and amount of waste material included in such a way as not to markedly worsen these basic attributes.
For example, U.S. Pat. No. 6,203,450 describes the compounding of polyurethane rubber in the core material. However, the resulting golf balls can hardly be said to have a suitable feel, and moreover are unable to achieve a reduced spin rate.
JP-A 2011-005329 discloses that, by granulating an ionomer resin which was used as a cover material and including the granulated resin in a core-forming rubber composition, it is possible to utilize such a waste material while suppressing a decline in ball performance.
Also, as is widely known, two-piece solid golf balls are composed of a core and a cover, with the core being a crosslinked rubber material of certain desirable properties obtained by using a base rubber composed primarily of cis-1,4-polybutadiene rubber to which compounding ingredients such as a co-crosslinking agent, a metal oxide and an organic peroxide have been added. For example, JP-A 59-49779 teaches a rubber composition for the core of a two-piece solid golf ball, which composition is obtained by compounding a given amount of zinc methacrylate as a co-crosslinking agent in cis-1,4-polybutadiene rubber. However, when zinc methacrylate is used in this way in a core-forming rubber composition, achieving good ball durability in long-term use is difficult.
In addition, JP-A 2003-70936, JP-A 2007-61614, JP-A 2007-301357, JP-A 2010-115485, JP-A 2010-115486, JP-A 2004-180793, JP-A 2008-149190, JP-A 2009-195761, JP-A 2005-27814, and JP-A 2010-269147 all describe, as rubber compositions for the cores of two-piece solid golf balls, the compounding of given amounts of zinc acrylate in cis-1,4-polybutadiene rubber. However, here too, when zinc acrylate is used in the core-forming rubber composition, achieving good ball durability in long-term use has been difficult.
Prior art relating to this invention is described in JP-A 2002-126128, which is directed at a one-piece golf ball having an optimized internal hardness profile from the surface toward the center of the ball. However, this prior-art golf ball lacks a satisfactory durability to “surface loss” when hit with the sharp portion of a clubhead, such as the leading edge of an iron.